Having read several road tests on-line, it seems my Lexus GS is prone to twitchy steering at low speeds. This is due to the variable-ratio, speed-sensitive, electronic power steering, which gives a high ratio at low speeds and a lower ratio at high speeds. But whilst the high ratio is great for parking, it does make the car a bit twitchy at town speeds. This is supposed to disappear the faster you go, but I haven't been on a motorway with it yet to see if that's right. The twitchiness probably isn't helped by the wheel/tyre combination, but it would seem that this is not the root cause.